1. Field of the Invention
The present invention relates to improvement in apparatus and method of processing a video signal, which is capable of converting an interlace video signal into a non-interlace video signal having a number of scanning lines double the original number of scanning lines in one field.
2. Description of the Related Art
As well known, in recent years, the display performance of television receivers has been improved, with larger-sized television screens and better definition (high image quality). As a result, a problem related to image quality, which was not conspicuous before, has arisen. For example, when a television receiver displays an interlaced video signal, line flicker and large-sized screen flicker are sometimes conspicuous.
For this reason, the interlace video signal is displayed after being converted into a non-interlace video signal. In this case, the non-interlace video signal is generated in the following manner. That is, double speed conversion is made with respect to an interlace video signal, using an image memory so that the number of scanning lines in one field is double the original number of scanning lines. However, according to the above method, although no line flicker or large-sized screen flicker are generated, there is the problem of deterioration (reduction) in vertical resolution, due to the scanning line interpolation method.
In order to solve the above problem, there has been conventionally developed the following technique of reducing the deterioration of image quality, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 63-179685, for example. According to the above technique, the luminance level of the interpolation signal is changed in accordance with the luminance level of the original video signal, and thereby, a visual contrast feel is improved between the scanning lines.
Meanwhile, in order to improve the above visual contrast feel, the television receiver is provided with a black stretching circuit, which reduces a luminance level lower portion of the video signal to a black signal level or less, regardless of the following television receiver types. The types include a type displaying only an interlace video signal, and a type capable of displaying interlace and non-interlace converted video signals.
An inputted interlace video signal is converted into a non-interlace video signal by generating an interpolation signal generated from each scanning line of the inputted video signal. In this case, the black stretching circuit has a function of carrying out black stretching with respect to the generated interpolation signal, as a function of varying the luminance level.
In the non-interlace video signal having the black stretched interpolation signal generated by the scanning line interpolation, black stretching is further carried out with respect to each scanning line of the original video signal. Thereafter, the video signal is image-displayed by a display section via a display drive circuit.
Incidentally, in the above television receiver, a scanning line interpolating circuit carries out black stretching with respect to an interpolated scanning line in order to prevent a deterioration of vertical resolution resulting from the scanning line interpolation. For this reason, the latter-stage of the scanning line interpolating circuit, that is, a black stretching circuit detects an already black-stretched portion of the interpolation signal outputted from the scanning line interpolating circuit.
In this case, in the black stretching circuit, when many black-stretched portions exist in the preceding scanning line, an internal time constant is set so that a response time for carrying out black stretching with respect to a video signal of the next scanning line becomes late. As a result, there is a problem that sufficient black stretching is not carried out with respect to the original video signal, which should be black-stretched.